The present invention is related to the field of fiberoptic systems and networks in which signals are directed from an optical source to a receiver by the state of polarization and the wavelength of the optical signal. Such systems and networks combine polarization-division multiplexing (PDM) and wavelength-division multiplexing (WDM) to increase the capacity, i.e., bandwidth, of the system or network for carrying information.
As modern telecommunication and data communication systems and networks have increasingly required more and more bandwidth, fiberoptic technology has come to the fore. One area of exploration has been the use of the state of polarization of a light signal to carry information. Lithium niobate (LiNbO.sub.3) phase modulators and polarization controllers, and Pockels cells have been used to implement polarization phase-shift keying (POLSK). Other investigations have considered the polarization of signals in combination with time slots for different communication channels to propose polarization division multiplexing/time domain multiplexing communication systems.
However, for one reason or another, in recent years efforts to use the polarization of light signals have fallen into disfavor. Technologies, adapted from the electrical field, such as code division multiplexing (CDM), have been used in fiberoptic systems and networks. A purely optical technology, wavelength-division multiplexing (WDM), has become very popular. In a WDM system or network, the wavelength of an optical signal guides the signal from its source to its intended destination. To increase the bandwidth of such systems and networks, the so-called Dense WDM (DWDM) standard has been proposed. Under DWDM, the channel separation of signals is set at 0.8 nm in wavelength, or 100 GHz in frequency, so that more communication channels may be created.
Nonetheless, the transmission capacity of cost-effective CDM and WDM systems is limited, since the number of available channels is limited. For instance, it is expensive to select or produce laser diode sources at different wavelengths with the required characteristics in DWDM systems. Telecommunication and data communication systems are expected to require bandwidths of terabits per second, or even petrabits per second.
Hence, it is still highly desirable that the bandwidth of a communication system be increased and increased at reasonable cost. The present invention achieves this goal by utilizing the state of polarization of a light signal in WDM systems and networks. The present invention also permits an increase in the number of users for various types of fiberoptic network systems.